Heretofore, as a ball for a ballpoint pen, there has been used a ball such as a zirconia ball or a cemented carbide ball containing zirconium oxide, tungsten carbide, or the like as a main ingredient and further a binder composed of a metal.
Since a metal such as cobalt, chromium, titanium, or nickel is employed as the binder in the above ball, in the case where the ball is used in a water-based ballpoint pen, the binder is dissolved into an ink with the passage of time by the action of a corrosive ingredient (e.g., dissolved oxygen) in the ink. Thus, when the binder is removed from the ball, crystal particles of zirconium oxide, tungsten carbide, or the like as the main ingredient may fall away and/or the binder dissolved may be transformed into a metal oxide and insolubilized to adhere again to the surface of the ball, so that the ball sometimes becomes in a so-called corroded state. Since irregularities of the ball surface are increased by the corrosion, there are sometimes arise such problems that the rotation of the ball is inhibited to result in a heavy feel while writing, and a smooth flow of the ink is inhibited to cause blur in handwriting.
In particular, among the cemented carbide balls using tungsten carbide as a main ingredient, since a ball using cobalt or nickel as a binder has a large content of the binder, it has a defect that the ball tends to be corroded with the passage of time.
As a method of preventing the aforementioned corrosion with the passage of time, there has been disclosed a method of adding a sulfide compound into an ink (see, e.g., Patent Document 1).
[Patent Document 1] Japanese Patent No. 3838183